1. Field of Invention
The present invention relates generally to an image sensor package and a fabrication method thereof. More particularly, the present invention relates to an image sensor of a quad flat non-leaded package (QFN) and a fabrication method thereof.
2. Description of the Related Art
During the past few years, photography in all its forms has been used more frequently. Integrated circuits (IC) and other sophisticated electronics are now commonplace in cameras of all kinds and sizes. Since consumers expect more from their digital cameras and video products, digital imaging technology increasingly improves the quality of images and reduces product costs. The image sensors comprise charge-coupled devices (CDD) and complementary metal oxide semiconductor (CMOS) digital image sensors, for example. CDD and CMOS digital image sensors receive external light and transfer it into digital signals. Therefore, those digital image sensors are expected to deeply penetrate the high-tech market.
Since image sensors need to absorb light, the packaging method of the image sensor is different from the packaging method of a general electronics product. U.S. Pat. No. 5,523,608 and U.S. Pat. No. 5,529,959 disclose a package structure that utilizes a plastic quad flat package (PQFP) to package an image sensor. However, the conventional method provides a large size package structure; thus the conventional package structure is not suitable for a high-density package. U.S. Pat. No. 5,898,218 and U.S. Pat. No. 5,270,491 describe a conventional method of utilizing a ceramic carrier, such as a ceramic leadless chip carrier (LCC), to package the image sensor. Although LCC has a better hermetically sealed characteristic and is highly durable, the cost of the fabrication is very high and the fabricating process is complicated. Thus, the ceramic carrier is not suitable for a low-cost packaging market. Another conventional method is disclosed in U.S. Pat. No. 5,811,799, which utilizes a laminated substrate as a carrier for a plastic package of an image sensor. The laminated substrate is formed as a non-leaded carrier. Although the size of this type of package can be reduced, the cost is higher because the carrier uses a material of flame-retardant epoxy-glass fabric composite resin, FR-4, FR-5, or bismaleimide-triazine (BT).
The image sensor package normally comprises a transmittance lid, molding structure, conductive wires, a carrier and an image sensor. In order to allow the image sensor to receive light, a good transmittancy between the active region of the image sensor to the transmittance lid is required. A conventional method to improve the transmittancy is to use a glass lid or a transmittance plastic lid. Glass has better transmittancy. The air between the image sensor and the transmittance lid is vacuumed out so that an air expansion inside the device can be prevented when the device is subjected to heat. Thus, a highly sensitive image sensor package usually utilizes a vacuum method. However, another method is suggested by filling a space between the image sensor and the transmittance lid with a transmittance material or epoxy in order to replace the vacuum method. When the transmittance material or epoxy is used to fill the space between the transmittance lid and the image sensor, the transmittancy of the transmittance material is not as good as the vacuum status. A transmittance material that has high temperature resistance and high inertness properties, but it is very costly; thus, this method is not effective and is not widely used.
Since image sensors are generally utilized on portable products such as a digital camera, or even an image capturing device of a telephone in the future, image sensor devices require shock resistance. Tests performed on the fabrication process become very important to ensure producing high quality of image sensors. The reliability test and the centrifugal test are usually performed during the fabrication process. Conventional testing equipment disclosed in U.S. Pat. No. 5,523,608 uses only a thin layer of molding compound to encapsulate leads and a die pad. During the reliability test, the package structure is detached due to the poor bonding. If the length of the conductive wire is too long, it will be deformed during the centrifugal test; thus, the reliability of the package is reduced.
It is an object of the present invention to provide an image sensor of a quad flat non-leaded package (QFN), comprising: a lead frame having a plurality of leads and a die pad. The leads are located around a periphery of the die pad, and the lead frame has a first surface and a corresponding second surface, each lead having a bonding portion. A molding structure is formed around an outer boundary of the leads and located on the first surface of the lead frame. The molding structure and the lead frame are arranged in such a way that a space is formed. A chip has an active surface and a corresponding back surface. A plurality of bonding pads is formed on the active surface, and the chip is located in the space and the back surface of the chip is adhered onto a first surface of the die pad. A plurality of wires is utilized to electrically connect the bonding pads respectively to the bonding portions of the leads on the first surface of the lead frame. A liquid compound is filled in between the chip and the molding structure, covering portions of the wires. A surface of the liquid compound is approximately coplanar to the active surface of the chip. A transmittance lid is allocated over the active surface of the chip to seal the space.
It is another object of the present invention to provide a carrier having an insulating body and a plurality of leads. The leads are fixed in the insulating body and a cavity is formed in the insulating body. An image sensor chip has an active surface and a corresponding back surface. A plurality of bonding pads is formed on the active surface of the chip, and the back surface of the chip is adhered onto a bottom surface of the cavity. A plurality of wires, electrically connect the bonding pads respectively to the leads. A liquid compound is filled in between the chip and the insulating body, covering portions of the wires. A top surface of the liquid compound is approximately coplanar to the active surface of the chip. A lid is located over the active surface, sealing the cavity.
A further objective of the present invention is to provide a method of fabricating an image sensor of a QFN, comprising the following steps. A lead frame having a plurality of package units is provided. Each package unit has at least a die pad and a plurality of leads. The leads are formed around a periphery of the die pad, and each lead has a bonding portion. The lead frame further has a first surface and a corresponding second surface. A molding process is performed, and at least a molding structure is formed on the first surface of the lead frame. The molding structure surrounds an outer boundary of the leads and covers gaps between the die pad and the leads. A first surface of the die pad and the bonding portions of the leads are exposed. The lead frame and the molding structure of every package unit are arranged in such a way that a space is formed for locating a chip. A plurality of chips is provided, wherein each chip comprises an active surface and a corresponding back surface. A plurality of bonding pads is formed on the active surface of each chip. A die attaching process is performed, each chip is placed respectively in the space, and the back surface of the chip is adhered onto the first surface of the die pad. A wire bonding process is carried out to utilize wires to electrically connect the bonding pads to the bonding portions of the leads.
This is followed by performing a dispensing process, wherein a liquid compound is filled in between the chip and the molding structure and covered portions of the wires. A top surface of the liquid compound is approximately coplanar to the active surface of the chip. A transmittance material is located over the active surface of the chip to seal the space. A singulating process is performed to separate the package units.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed.